However, due to “double fading” effect (i.e., the total path loss of the cascaded transmitter-RIS-receiver link is the product of the path losses of the transmitter-RIS link and RIS-receiver link, which is usually an order-of-magnitude larger than that of the direct link), RIS should be deployed close to either the base station (BS) or users to effectively improve communication performance [1]}-[2]}. Besides, recent studies showed that RISs can be easily outperformed by conventional full-duplex (FD) amplify-and-forward (AF) relay unless very large RISs are employed [3]}, [4]}. In [3]}, the authors compared the RIS with classic decode-and-forward (DF) relaying and discovered that very large meta-surfaces are needed to surpass DF relaying.
The authors in [4]} carried out a performance comparison in terms of achievable rate and energy efficiency. Numerical results showed that the RIS-aided system outperforms the relay-aided network only for adequately large RIS. However, massive reflecting elements will lead to high training overhead for channel estimation [7]}, [8]} and high power consumption to maintain the circuit operations. Consequently, the control of RIS will also become more difficult [9]}. Therefore, the purely passive RIS might have limited application scenarios in practical wireless systems.
